No one's arguing that DuMorne isn't D-E-D dead. The theory is that when Kemmler went down the last time, he took over DuMorne's body a la Corpsetaker/Butters. This left Dumorne dead, and Kemmler is now known as Cowl. 

i forget wich book its in but they talk about justin being one of the wardens at kemmlers final stand. its also mentioned that he changed after. mabe harry didnt kill him but he took the bob and im sure justin was kemler triing to rebuild his army. by morgana i ment morgana lafae theres and kemler keeps getting brought up over and over hes not dead. if you go over old aurthurian is that the right word legends morgana was a necromancer she brought mordred back. or i could just be crazy

Yeah, I put a theory up about that earlier. I think Kemmler took over DuMorne when he (Kemmler) was killed (again) a la Corpsetaker/Butters. We know from Luccio that strength in magic is relative to the physical body (She is weaker than she was before, and can no longwr make swords).  I think DuMorne was the strongest body available at that moment. Kemmler then found Harry and Elaine, maybe planning to seize one of them. Harry escaped, and either overpowered DuMorne because he was that much stronger, or because he got help from Lea (I think this might have been mentioned in other books that she gave him power in exchange for a favor - I don't have the books handy to look that up). Anyway, I believe that Cowl is Kemmler, resurrected again.